ENH: copy everything in src/ext/include/ to public/ on make install

Summary

This allows to include separate web applications.

Test Environment

How to set up a test environment for manual testing?

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• All automated tests pass
• Reference related Issues
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  • Intent of new code
  • Problems with old code
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• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
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Makefile

@@ -176,6 +176,9 @@ cp-ext:
 	for f in $(wildcard $(SRC_EXT_DIR)/xsl/*) ; do \
 		echo "y" | $(CMD_COPY_EXT_FILES) "$$(realpath "$$f")" $(PUBLIC_DIR)/xsl/ ; \
 	done
+	for f in $(wildcard $(SRC_EXT_DIR)/include/*) ; do \
+		echo "y" | $(CMD_COPY_EXT_FILES) "$$(realpath "$$f")" "$(PUBLIC_DIR)/$$(basename "$$f")" ; \
+	done
 
 cp-ext-test:
 	for f in $(wildcard $(TEST_EXT_DIR)/js/*) ; do \